Use a commercial insecticidal soap labeled for your indoor plant and identified pest, rather than assuming dish detergent or castile soap is the same product. Insecticidal soap is a contact treatment for susceptible pests; it is not a fertilizer, a leaf-shine spray or a lasting repellent.
The important questions are whether the pest is susceptible, whether the plant tolerates the product and whether the label allows the intended indoor use. If one of those is uncertain, get identification or product guidance before spraying.
What it can and cannot do
Insecticidal soaps contain fatty-acid salts and act on susceptible insects or mites through direct contact. They are commonly used against some soft-bodied pests. They do not provide a universal answer for every scale insect, egg or soil-dwelling larva.
Clemson Extension describes contact action and plant-sensitivity concerns. Once the spray dries, do not expect the same continuing control from its residue. Persistent pests may reflect missed insects, another life stage or an unsuitable treatment, not a need for an arbitrarily stronger mixture.
Read the label before preparing the plant
Confirm the plant, pest, indoor location and any exclusions. Note whether the bottle is ready to use or must be diluted. Follow the stated concentration and application interval; there is no safe universal weekly-for-four-weeks recipe for all products and plants.
Follow instructions about testing a small area, temperature, plant stress and coverage. Do not treat a sensitive plant merely because a different houseplant tolerated the spray. Avoid applying to wilted or otherwise stressed plants unless the label specifically permits the circumstances.
Apply only as directed
Protect household surfaces and keep people and pets away as instructed. Target the affected plant with the coverage the label requires, including accessible pest hiding places. Avoid drift onto food, aquariums, animal areas or untreated plants. Use the required protective equipment and ventilation.
Do not add cooking oil, fertilizer or another pesticide to improve sticking or strength unless the label permits that combination. Store the product securely in its original labeled container; do not use a drink bottle.
Why kitchen soap is not a reliable shortcut
Dishwashing detergents can include ingredients not intended for plant leaves, and household soap formulations differ. A small test cannot turn an unverified recipe into a guarantee of safety or effectiveness. Commercially formulated and labeled plant products provide information a kitchen mixture lacks.
Follow up with observation
Inspect the plant for live pests and any injury after treatment. Keep a dated record and repeat only as the label allows. If control fails, reconsider pest identification, coverage and plant condition with a local plant specialist. For fungus gnats, focus on the growing medium and moisture management rather than expecting a leaf spray to solve the whole problem.
